The present invention relates to a speed control apparatus for elevators, and more particularly to a speed control apparatus for elevators which prevents the burning of a motor winding during a low speed operation.
There has heretofore been an elevator speed control apparatus of the pertinent type as desclosed in the official gazette of Japanese Patent Application Laid Open No. 56-123795, and an arrangement block diagram thereof is shown in FIG. 1. Referring to the figure, an elevator is constructed of a rectifier 2 which receives an input from an A.C. power source 1 to convert it into direct current, a smoothing capacitor 3 which smooths the output voltage of the rectifier 2, an inverter 4 which includes a transistor and a diode and which inverts the D.C. voltage smoothed by the capacitor 3 into alternating current of variable voltage and variable frequency, an induction motor 5 which is driven by an A.C. source voltage inverted by the inverter 4, a sheave 6 which is driven by the rotation of the induction motor 5, a traction rope 7 which is wound round the sheave 6, a cage 8 which is coupled to one end of the traction rope 7, and a counterweight 9 which is coupled to the other end of the traction rope 7. In such elevator, the speed control apparatus for the elevator comprises a speed pattern generator 11 which generates a speed command signal 11a, a current detector 12 which detects the current to be supplied to the induction motor 5, a speed detector or tachometer generator 13 which detects the rotational speed of the induction motor 5, a control signal generating circuit 14 which compares and operates the respective outputs of the speed detector 13 and the speed pattern generator 11, a pulse width modulation (hereinbelow, termed "PWM") comparator 16 which compares the respective outputs of the control signal generating circuit 14 and the current detector 12 to generate a pulse width modulation command, and a base driving circuit 17 which generates a gate signal for the control of the transistor constituting the inverter 4 on the basis of the PWM command of the PWM comparator 16.
Here, the induction motor 6 usually employed is of the self-ventilating type. In a case where the elevator is being operated at its rated speed, the rotational frequency of the induction motor 5 is high, and hence, the cooling effect is kept high. In contrast, in a case where it is being operated at a low speed, particularly in a manual operation mode for installation, maintenance and inspection, or the like, the cooling effect worsens, and the winding of the induction motor 5 might burn when the low speed operation is continued for a long time.
The relationship between the rate of ventilation Q required for the cooling of the induction motor 5 and the thermal resistance R of the induction motor is indicated as follows: EQU Q=Q.sub.o .multidot.(N/N.sub.o) (1)
where
N.sub.o ; rated rotational frequency of the motor, PA1 N; rotational frequency of the motor, PA1 Q.sub.o ; rate of ventilation in the rated rotation operation. EQU R=R.sub.o .multidot.(Q.sub.o /Q).sup.0.4 to 0.5 ( 2)
where R.sub.o ; thermal resistance in the rated rotation operation.
From Eqs. [1] and [2] mentioned above, as the rotational frequency N lowers, the rate of ventilation Q decreases, and further, as the rate of ventilation Q decreases, the thermal resistance R increases. In this manner, the lowering of the rotational frequency increases the thermal resistance, resulting in the disadvantage that the motor winding burns.
In order to avoid the disadvantage, the prior art has set a limit to the period of time of the manual operation so as to cope with the burning. With this measure, however, when the elevator is operated under a heavy load in excess of a predetermined value beyond the limit period, the burning of the motor winding is still feared to occur.